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UTC is a combination of TAI and yet another time scale named UT1.

UT1 is a measure of the rotation angle of planet Earth relative to the direction from the Earth to the Sun. Each UT1 day has 24 UT1 hours, each UT1 hour has 60 UT1 minutes, and each UT1 minute has 60 UT1 seconds. It is noon in UT1 when Greenwich is under the Sun. At the time of writing, UT1 is around 34 seconds behind TAI. In 1972, UT1 was 10 seconds behind TAI.

As mentioned, UTC is a combination of TAI and UT1. UTC equals TAI plus a politically decided offset. This UTC offset indicates how much UTC lacks behind TAI. At the time of writing, the UTC offset is 34 seconds indicating that UTC is 34 seconds behind TAI.

At any time, the UTC offset is an integral number of seconds, but the UTC offset may be incremented or decremented by decree from the International Earth Rotation Service (IERS). Hence, UTC depends on TAI and IERS, but IERS has the intension to keep the difference between UTC and UT1 below 0.9 seconds, so UTC indirectly depends on UT1.

UTC makes a leap whenever IERS increments or decrements the UTC offset. Such leaps are implemented by irregular UTC minutes.

A regular UTC minute has 60 UTC seconds. An irregular one either has 59 or 61. Apart from that UTC counts like TAI and UT1: days have 24 hours and hours have 60 minutes.

Whenever IERS increments (decrements) the UTC offset, the last minute of the last hour of a particular day has 61 (59) seconds. IERS intends to place irregular seconds at the end of June 30 and December 31 when necessary and intends to announce the leaps in advance.

In Logiweb, UTC hour hh, minute mm, and second ss is written UTC:hh:mm:ss. We have 00 <= hh <= 23, 00 <= mm < 59, and 00 <= ss <= 60. Occasionally, we shall use UTC:24:00:00 to denote UTC:00:00:00 on the following day. Decimal fractions of a second are written after a dot as in UTC:12:23:34.456 which denotes 0.456 seconds past UTC:12:23:34. The notation is compatible with ISO 8601 except that we prepend 'UTC:' to emphasize the use of Universal Coordinated Time.

At the time of writing, IERS has never decremented the UTC offset, but has incremented the UTC offset at the end of the following days:

GRD-1972-06-30 GRD-1972-12-31 GRD-1973-12-31 GRD-1974-12-31 GRD-1975-12-31 GRD-1976-12-31 GRD-1977-12-31 GRD-1978-12-31 GRD-1979-12-31 GRD-1981-06-30 GRD-1982-06-30 GRD-1983-06-30 GRD-1985-06-30 GRD-1987-12-31 GRD-1989-12-31 GRD-1990-12-31 GRD-1992-06-30 GRD-1993-06-30 GRD-1994-06-30 GRD-1995-12-31 GRD-1997-06-30 GRD-1998-12-31 GRD-2005-12-31 GRD-2008-12-31

Before GRD-1972-06-03, the UTC offset was 10 seconds.

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